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New Warnings on Climate Change

The main international scientific body assessing causes of climate change is closing in on its strongest statement yet linking emissions from burning fossil fuels to rising global temperatures, according to scientists involved in the process.

In fresh drafts of a summary of its next report, the group, the Intergovernmental Panel on Climate Change, has said that it is more than 90 percent likely that global warming since 1950 has been driven mainly by the buildup of carbon dioxide and other heat-trapping greenhouse gases, and that more warming and rising sea levels are on the way.

Some scientists involved in drafting the report confirmed and clarified details but asked not to be identified because it was not finished.

In its last report, published in 2001, the panel concluded that there was a 66 to 90 percent chance that human activities were driving the most recent warming.

The shift in language in the current draft, while subtle, is substantive. If it remains in the final version, scheduled for release in Paris on Feb. 2, it will largely complete a quest that lasted decades to determine if humans are nudging the earth’s thermostat in potentially momentous ways.

Drafts of the report project a most likely warming of 4 to 8 degrees if the concentration of carbon dioxide in the atmosphere rises to twice the 280 parts per million that it averaged for many centuries before the Industrial Revolution.

The carbon dioxide concentration is now roughly 380 parts per million, and many climate experts say it will be extremely difficult to avoid hitting levels of 450 or 550 parts per million, or higher, later this century, given growth in populations and fuel use and the lack of nonpolluting alternatives that can be exploited at a sufficient scale to replace fossil fuels.

Scientists involved in writing the panel's three massive scientific volumes and related documents declined to discuss the drafts because many changes could take place before the final version was published.

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Because the panel works under the auspices of the United Nations, dozens of officials from governments around the world have been critiquing drafts, and details inevitably begin to slip into the press in the weeks preceding the formal release.

Snippets of earlier drafts have leaked to some newspapers in recent months and some sections of the latest draft were first published in The Toronto Star yesterday.

Scientists involved in writing the report said the leaks were damaging and potentially misleading, mainly because the final statements are likely to go through further changes.

“The language is far from final,” said Kevin E. Trenberth, a senior scientist at the National Center for Atmospheric Research in Boulder, Colo., who is a lead author of one section. “You can’t say what the I.P.C.C. says until it actually says it.”

Jerry Mahlman, an emeritus researcher at the same center who was a reviewer of the report's single-spaced, 1,644-page summary of climate science, said that most of the leaks were from people eager to find elements that were the scariest or most reassuring.

He added in an interview yesterday that such efforts distract from the basic, undisputed findings, saying that those point to trends that are very disturbing.

He pointed to recent disclosures that there is still uncertainty about the pace at which seas will rise due to warming and melting of terrestrial ice over the next 100 years. That span, he said, was just the start of a process of a rise in sea levels that would then almost certainly continue for 1,000 years or so.